1. Field of the Invention
This invention relates to a method for manufacturing a photomask used in a photolithography process for manufacturing semiconductor devices and a method for manufacturing semiconductor devices by using the above photomask.
2. Description of the Related Art
Recently, extremely severe precision has been more strongly required for a photomask used in a photolithography process as semiconductor devices are more increasingly downsized. Further, the pattern of the photomask is subjected to optical proximity correction (OPC) and the number of patterns having complicated shapes is increased. Therefore, it is insufficient to ensure the quality of the photomask by a one-dimensional evaluation method such as a simple pattern dimension measurement method and it is required to use a two-dimensional pattern shape control method.
In order to meet the above requirement, the following method for checking whether or not a mask pattern is finished with desired precision is proposed. First, an image of a mask pattern is acquired by means of a scanning electron microscope (SEM) and a pattern shape after lithography is calculated by simulation based on pattern contour data extracted from the image. Then, whether or not a mask pattern is finished with desired precision is checked based on whether or not a desired degree of lithography margin can be attained by using the thus calculated pattern shape. The merit of this method is that the finishing degree of the mask pattern can be determined in a state extremely close to the condition in which the pattern is actually used and exposed onto a wafer. As a result, necessary and sufficient control can be realized without performing unnecessarily strict control or lax control.
For example, in Jpn. Pat. Appln. KOKAI Publication No. 2007-163686, a method attained by further expanding the above method to manage the finishing degree of a photomask is proposed. That is, a lithography simulation is run by deriving not only pattern contour data but also a 3-dimensional shape on the photomask based on a pattern image by means of an SEM.
Further, as a method for further advancing shrinkage, a multiple exposure technique for forming a pattern by using a plurality of photomasks for one layer (to-be-processed film) formed on a wafer is proposed. For example, in the double-exposure technique, first, photoresist is coated on a to-be-processed film, the photoresist film is exposed by means of a first photomask and developed to form a resist pattern and the to-be-processed film is patterned by using the resist pattern. Subsequently, photoresist is newly coated on the thus patterned to-be-processed film, the photoresist film is exposed by means of a second photomask and developed to form a resist pattern and the to-be-processed film is further patterned by using the resist pattern. Thus, the to-be-processed film is processed into a pattern that is a combination of the patterns of the first and second photomasks.
When the multiple exposure technique is applied, the finishing precision of a pattern formed on the to-be-processed film on the wafer is determined by the precisions of the respective photomasks used. Further, it becomes necessary to determine not only the precisions of the individual photomasks but also the quality thereof by taking the mutual interaction into consideration. That is, even if the photomask is low in precision in a singular form and the pattern becomes faulty, the pattern will become good by the mutual interaction between plural photomasks. For example, a line-form pattern defined by edge positions and distances between the edge positions is considered. When a pattern is formed by means of a first photomask with the dimension thereof reduced by Δ, the right-side pattern edge is moved to the left side by Δ/2 and the left-side pattern edge is moved to the right side by Δ/2. On the other hand, when a pattern is formed by means of a second photomask with the dimension thereof increased by Δ, the left-side pattern edge is moved to the left side by Δ/2 and the right-side pattern edge is moved to the right side by Δ/2. In the above cases, if Δ is large, the first and second photomasks become faulty in the singular form. However, if a multiple exposure process is performed, the deviation amounts thereof cancel each other to set the relative distance between the pattern edges to a normal value and the pattern dimension is set to a desired value.
On the other hand, even if the precision of the photomask is sufficiently high in the singular form, deviations of plural photomasks may be accumulated and the deviation amount may be increased by the mutual interaction between the plural photomasks, and as a result, the sufficiently high precision cannot be attained in some cases.
Thus, when the multiple exposure technique is used, it is necessary to take the mutual interaction between the plural photomasks used for patterning the same layer (to-be-processed film) on the wafer at the time of quality control of a to-be-formed pattern. Therefore, a satisfactory result cannot be attained simply by determining whether the pattern is good or not by a lithography simulation based on pattern images acquired by means of individual photomasks.
Thus, it is desired to provide a photomask manufacturing method capable of enhancing the dimensional precision on a wafer while avoiding occurrence of faults after multiple exposure and a semiconductor device manufacturing method using the above photomask.